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Reinstating the M31 X-ray system RX J0042.3+4115 as a black hole X-ray binary, and compelling evidence for an extended corona

The M31 X-ray source RX J0042.3+4115 was originally identified as a black hole binary because it displayed characteristic low state variability at conspicuously high luminosities; unfortunately, this variability was later found to be artificial. However, analysis of 84 Chandra ACIS observations, an HST ACS/WFC observation, and a 60 ks XMM-Newton observation has supplied new evidence that RX J0042.3+4115 is indeed a black hole binary. The brightest optical star within 3 sigma of the position of RX J0042.3+4115 had a F435W (~B) magnitude of 25.4+/-0.2; M_B > -0.4, hence we find a low mass donor likely. RX J0042.3+4115 was persistently bright over ~12 years. Spectral fits revealed characteristic black hole binary states: a low/hard state at 2.08+/-0.08 E+38 erg/s, and a steep power law state at 2.41+/-0.05 E+38 erg/s (0.3--10 keV). The high luminosity low state suggests a ~20 M_Sun primary; this is high, but within the range of known stellar black hole masses. The inner disk temperature during the steep power law state is 2.24$\pm$0.15 keV, high but strikingly similar to that of GRS 1915+105, the only known Galactic black hole binary with a low mass donor t o be persistently bright. Therefore RX J0042.3+4115 may be an analog for GRS 1915+105; however, other mechanisms may account for its behavior. We find compelling evidence for an extended corona during the steep power law state, because compact corona models where the seed photons for Comptonization are tied to the inner disc temperature are rejected.